Producing asteriated corundum crystals



Patented Oct. 5, 1954 PRODUCING ASTERIATED GORUNDUM CRYSTALS William G. Eversole and John N. Burdick, Ken? more, N. Y., assignors to Union Carbide and Carbon Corporation, a corporation of New York No Drawing. Application December 28, 1951, Serial NO. 263982 17 Claims.

This invention relates to a novel method of producing asteriated crystals of corundum, such as ruby and sapphire of various colors, for ex ample garnet-colored, blue, white, green, yellow, brown, and alexandrite-like sapphire. The meth d has been used for creating asterism in unasteriated corundum crystals, as Well as for improving poorly asteriated corundum crystals. Furthermore, the invention is also concerned with novel asteriated corundum crystals resulting from the novel method.

In accordance with the present invention our novel method involves heating a single crystal of corundum at a tempreature between 1700 and 150 C. while maintaining a titanium compound in contact with the surface of the crystal to introduce some of the titanium compound into the solid crystal. Then the crystal is heated at a temperature between 1100" and 1500 C. to develop asterism by causing tiny rutile (TiOz) crystals to precipitate in the host corundum crystal, as described in U. S. Patent 2,488,507.

This method is operable on any shape of corundum crystal, such as rough natural crystals, or whole and half-boules of synthetic corundum, but the greatest economies of material and cost and the most desirable products are obtained by treating dome-shaped cabochon gemstones which have been cut from larger crystals of natural or synthetic corundum. To exhibit a symmetrical 6-rayed star, cabochon gemstones must be properly shaped with a convex dome-shaped crown, and with the crystallographic C-axis extending through the crown parallel to the geometric axis. A gemstone of the cats-eye type, exhibiting only a single ray bisecting the crown, can be made by cutting the cabochon in such a way that the C-axis extends perpendicularly to the geometric axis of the cabochcon. Such cabochon gemstones can be cut from half-boules or annealed whole boules of any crystallographic C-axis orientation between zero and 90, or from natural corundum crystals.

An important advantage of the invention is that a manufacturer who has a market for both plain and asteriated synthetic corundum gemstones can now grow a single lot of boules of a desired color, and then follow the process of the invention for asteriating a portion of the lot. Formerly it was necessary to grow two lots of boules, one of which contained sufiicient titania for asterism and was consequently somewhat more difficult and expensive to grow than the other.

More specifically in accordance with the invention, titania (titanium dioxide) alone "can be placed in contact with a corundum crystal, or it can be mixed with a high-melting material such as alumina to reduce the fluxing action of the relatively low-melting titania and allow the use of higher treatment temperatures than would otherwise be convenient. Mixtures containing between 10% and 90% by weight of titania, balance alumina, have been used successfully, but alumina is also advantageous in smaller amounts up to 10%.

A preferred procedure for maintaining a titanium compound in contactwith the surface of a corundum crystal during heating is to place titania in contact with the crystal surface by burying one or more crystals in a dry powder comprising titania contained in a crucible. For example a mixture of 35 parts titania powder and parts alumina powder is calcined 4 hours at 1300 C. This calcined mixture is then crumbled and the finely-divided grains (which may be 5-25 microns in diameter) are placed in the crucible for receiving the corundum crystals to be treated.

Titania has also been placed in contact with the crystal by first forming a thick creamy water slurry comprising finely-divided titania, either alone or mixed with alumina, and then spraying a coating of this slurry on the surface of the crystal to be asteriated. The water evaporates and leaves a dry coating on the crystal surface. Thereafter the crystal is heated in a furnace to cause diffusion of the titania into the crystal, followed by the lower temperature heat treatment for developing asterism by precipitating tiny rutile crystals out of solid solution. Another suitable way for maintaining a titanium compound in contact with the crystal surface is to place the crystal in the furnace and then apply molten titania to the crystal While it is being heated, to cause diffusion of the titania into the crystal. The asteriating heat treatment follows.

While the diffusion of titania into the corundum crystal surface occurs at any temperature in the range between 1700 and 1950 C., the optimum combination of the greatest titania penetration in the shortest time without excessive melting of the alumina-titania 'mix and crust formation on the crystal is obtained at about 1800 C.

Experience has shown that the di'flusion rate of titania into the crystal varies as an inverse function of the temperature so that the time of treatment should be greater at lower temperatures than at higher temperatures. In general, one minor more man -temperature in the range of 1700 to 1950 C. will produce some asterism in a crystal, although the asteriated layer may be very thin at the lower temperatures and the shorter treatment times. For best results it is recommended that the treatment be extended for between 8 and 24 hours to develop a fairly thick asteriated skin of the order of 0.004 to 0.01 inch, which is thick enough to assure the final presence of asterism after finishing and polishing the surface.

Since blue sapphire tends to fade in color when heated in the 1'700-1950 0. range, it is advantageous to reduce fading to a minimum by maintaining an atmosphere of a reducing gas in the furnace, as by directly heating the furnace with an oxy-propane fiame having an excess of propane over that burned by the oxygen.

The following specific examples of how the method has been successfully performed illustrate the principles of the invention.

Example 1 A small boule of clear white synthetic sapphire was grown from pure alumina powder by the Verneuil process, whose general principles are disclosed in U. S. Patent 988,230 and in Annales de Chimie et de Physique Serial No. 3, Tome 3, page 20 (1904). Titania powder was then placed in the powder hopper and dispensed on to the completed boule as molten titania over a 6-hour period while holding the boule furnace at a temperature of about 1900 C. by means of the oxyhydrogen flame. The boule was removed and heat treated in another furnace at a temperature of 1300" C. for 72 hours. Strong asterism was found on the lower portion of the boule surface.

Example 2 A series of experiments was carried out in which a water slurry containing titania was sprayed on to the convex crowns of synthetic white sapphire and ruby cabochons weighing from 1 to 30 carats which had been cut from boules containing no titania. The cabochons were then placed in a gas-fired furnace heat treatedsome at temperatures of 1800 (3., others at 1825 C., 1850" C., and 1075" C. At each temperature cabochons were treated which had been sprayed with three different slurries made up from powders comprising 100%, 38%, and titania by weight, the balance being alumina. Subsequently all cabochons were given an asteriating heat treatment by lowering the furnace temperature to 1300 C. and holding the cabochons at this temperature for '72 to 96 hours. The cabochons were then removed from the furnace and polished. The following results were obtained:

A. All cabochons which had been treated one hour at 1875 C. had a thin asteriated layer in their crowns. A large part of the titania was found to have melted, run to the base, and thence diffused up into the cabochon. The resulting stars were rather diffuse because of light being reflected by the precipitated rutile from the base up to the crown of the cabochon.

B. In all cabochons which had been treated 24 hours at 1850 C. stars were successfully obtained. In this treatment a thin asteriated layer was obtained on the cabochon crowns. Part of the titania also melted, collected at the bases of the cabochons, and diffused upwardly, resulting in a final product having a rather diffuse star.

C. All cabochons treated 24 hours at 1825 C. were asteriated and exhibited stars. Part of the 4 titania had melted and run to the base of the stone as in Examples 2-A and 2-B.

D. After 24 hours at about 1800 C. the cabochons were asteriated over the whole crown to a depth of about 0.005 inch. Only the titania powder melted off the crowns of the cabochons appreciably.

E. After 12 hours of 1800 C. stars were obtained, although penetration of asterism was not as deep as in Example 2-D.

In comparing the results obtained with the three different slurry compositions, it was found that the coating containing 100% titania melted and ran off the cabochon crowns considerabl at all the temperatures. The coatings containing 38% and 10% of titania, balance alumina, remained on the cabochons during the furnace treatment at 1800 C. in what appeared to be the original condition; at 1825 C. there was partial melting. The 38% titania composition gave a somewhat more uniform penetration along the cabochon surface than the others and is therefore preferred. It is apparent, therefore, that best results can be expected when the coating composition is 30-40% titania by weight, balance alumina.

Example 3 A synthetic blue star sapphire cabochon which had been grown and asteriated as described in U. S. Patent 2,488,507 had a poor asymmetrical star and poorly distributed milkiness. It was embedded in a matrix of granular titania. filling an alumina crucible. The cabochon was heated in the crucible at 1950 C. for 12 hours and thereafter was polished and then given an asteriating heat treatment at 1300" C. to develop asterism. A symmetrical star and a well-distributed milky surface were obtained.

Example 4 Cabochons of pale blue-green unasteriated natural sapphire, of synthetic ruby, and of a garnetcolored, green, blue, yellow, brown, and alexandrite-like (one color in daylight, another color in artificial light) synthetic sapphire were embedded in a crumbled calcined mixture of 35 parts titania and 65 parts alumina in crucibles, as described previously. After heating the crucibles and contents at 1800 C. for from 24 to 30 hours the cabochons were removed from the mixture and where then asteriated by heating them at 1300" C. The crowns of the cabochons were then polished and all exhibited well-defined G-rayed stars. These cabochons were characterized by having a precipitate of rutile therein distributed substantially uniformly throughout a continuous thin skin on both the crown and base, the center portion of the mass of the cabochon within the skin being substantially free from precipitated rutile.

What is claimed is:

1. A method of asteriating a single crystal. of corundum which comprises holding such a crystal at a temperature between 1700 and 1950 C. while maintaining titanium oxide in contact with the surface thereof, to introduce some of said titanium oxide into said crystal; and then holding said crystal at a temperature between 1100 and 1500 C. to develop asterism therein.

2. A method of asteriating a single crystal of corundum which comprises placing titania in contact with the surface thereof and holding such crystal at a temperature betwene 1700 and 1950 C. for at least one hour to introduce some of said titania into said crystal, the length of the holding period varying as an inverse function of the temperature; and then holding said crystal at a temperature between 1100" and 1500" C. until rutile precipitates in said crystal to develop asterism therein.

3. A method of asteriating a single crystal of corundum which comprises placing titania in contact with the surface thereof and holding such crystal at a temperature of about 1800" C., to introduce some of said titania into said crystal; and then holding said crystal at a temperature between 1100" and 1500" C. to develop asterism therein.

4. A method of asteriating a single crystal of corundum which comprises placing a mixture of alumina and titania in contact with the surface thereof and holding such crystal at a temperature between 1700" and 1950" C., to introduce some of said titania into said crystal; and then holding said crystal at a temperature between 1100" and 1500" C. to develop asterism therein.

5. A method in accordance with claim 4 wherein said mixture comprises between 30% and 40% of titania and between 60% and 70% of alumina.

6. A method of asteriating a single crystal of corundum which comprises placing titania. in contact with the surface thereof, inserting such crystal in a furnace, and heating such crystal at a temperature between 1700" and 1950" C. to introduce some of said titania into said crystal, the length of the heating period varying as an in-- verse function of the temperature; and then reducing the temperature of said furnace to the temperature range between 1100" and 1500" C. and heating said crystal in said range for more than 2 hours to develop asterism therein.

7. A method of producing a star corundum gemstone which comprises providing a unicrystal line corundum cabochon having a crown, and having the C-axis thereof extending through said crown parallel to the geometric axis thereof placing titania in contact with said crown; then holding said cabochon at a temperature between 1700" and 1950 C., to introduce some of said titania into said cabochon; and thereafter holding said cabochon at a temperature between 1100" and 1500" C. to develop a G-rayed star therein.

8. A method of producing a cats-eye-like corundum gemstone which comprises providing a unicrystalline corundum cabochon having a crown, and having the C-axis thereof extending perpendicular to the geometric axis thereof; placing titania in contact with said crown; holding said cabochon at a temperature between 1700" and 1950" C., to dissolve some of said titania in said cabochon; and thereafter holding said cabochon at a temperature between 1100" and 1500" C. to develop a single ray therein bisecting said crown.

9. A method of asteriating a corundum crystal which comprises applying a slurry comprising a mixture of alumina and titania to a surface of said crystal; holding said crystal at a temperature between 1700" and 1950" C., to introduce some of said titania therein; and thereafter holding said crystal at a temperature between 1100" and 1500 C. to develop asterism therein.

10. A method of asteriating a single crystal of corundum Which comprises burying said crystal in a granular composition comprising titania; holding said crystal and said composition at a temperature between 1700" and 1950" C., to introduce some of said titania into said crystal; and thereafter holding said crystal at a temperature between 1100" and 1500" C. to develop asterism therein.

11. A method in accordance with claim 10 wherein said crystal is buried in a granular mixture of titania and alumina.

12. A method of treating a single crystal of corundum to place it in condition for asteriation which comprises heating such a crystal at a temperature between 1700" and 1950" C. for at least one hour while maintaining titanium oxide in contact with the surface thereof, to introduce some of said titanium oxide into said crystal, the length of the heating period varying as an inverse function of the temperature.

13. A method of treating a single crystal of corundum to place it in condition for asteriation which comprises placing titania in contact with the surface thereof, and heating such crystal at a temperature of about 1800" C. to introduce some of said titania into said crystal.

14. A method of treating a single crystal of corundum to place it in condition for asteriation, which comprises placing in contact with the surface thereof a mixture of alumina and titania, and heating such crystal at a temperature between 1700" and 1950" C. for at least one hour to introduce some of said titania into said crystal, the length of the heating time varying as an inverse function of the temperature.

15. A method in accordance with claim 14 where said mixture comprises between 30% and 40% of titania and between 60% and of alumina.

16. A method of treating a single crystal of corundum to place it in condition for asteriation which comprises applying a slurry comprising a mixture of alumina and titania to a surface of said crystal, and then heating said crystal at a temperature between 1700 and 1950" C. to introduce some of said titania therein.

17. A method of treating a single crystal of corundum to place it in condition for asteriation which comprises burying said crystal in a granular composition comprising titania, and then heating said composition and said crystal at a temperature between 1700" and 1950" C. to introduce some of said titania therein.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,814,219 Jaeger et al July 14, 1931 2,090,240 Strothman Aug. 17, 1937 2,270,210 Barbieri Jan. 13, 1942 2,270,270 Clare Jan. 20, 1942 2,330,193 Blau Sept. 28, 1943 2,488,507 Burdick et al. Nov. 15, 1949 2,498,003 Peterson Feb. 21, 1950 2,515,790 Navias July 18, 1950 

1. A METHOD OF ASTERIATING A SINGLE CRYSTAL OF CORUNDUM WHICH COMPRISES HOLDING SUCH A CRYSTAL AT A TEMPERATURE BETWEEN 1700* AND 1950* C. WHILE MAINTAINING TITANIUM OXIDE IN CONTACT WITH THE SURFACE THEREOF, TO INTRODUCE SOME OF SAID TITANIUM OXIDE INTO SAID CRYSTAL; AND THEN HOLDING SAID CRYSTAL AT A TEMPERATURE BETWEEN 1100* AND 1500* TO DEVELOP ASTERISM THEREIN. 